Method and apparatus for drilling holes with sub-wavelength pitch with laser

ABSTRACT

A method of laser machining using an ultra-fast pulse laser is presented. According to the present invention a plurality of holes with a pitch less than the wavelength of the laser are drilled into a material sample. Reliable and reproducible hole drilling is accomplished through an exemplary drilling sequence which applies a number of pulses at a first pulse energy to the surface spaced to avoid laser hardening of the surface for adjacent holes of a first set of holes. Next, the number of pulses is increased or the energy of the laser beam is increased to drill holes that are interstitial to the first set of holes. The exemplary laser machining process may used to produce both one-dimensional and two-dimensional photonic crystals, among other applications.

This Application claims benifit of Provisional Application Ser. No.60/171,894 filed Dec. 23, 1999.

BACKGROUND OF THE INVENTION

Many emerging material processing applications in the semiconductor andcommunications fields require sub-micron processing capability. A numberof competing technologies exist that either have, or will soon have,this capability, such as; electron beam etching, plasma: etching, x-raylithography, and machining with ultrafast pulse lasers (lasermachining). Of these technologies, only laser machining provides theadvantages of operation in a standard atmosphere and in situ monitoring.

An important feature of ultrafast pulse lasers is their capability toablate surface regions smaller than their minimum, diffraction limited,spot size. This capability is created by the brevity of the pulse, whichallows for essentially no spreading of heat during the pulse, and theGaussian spatial beam profile. By carefully controlling the energy of apulse, it is possible to raise the intensity of only a small region inthe center of the beam above the ablation threshold for the materialbeing machined. Because of the lack of heat conduction in the pulseduration, only the small region is ablated. In this way, holes may evenbe laser machined with diameters less than the wavelength of the laser,for example holes having a diameter of approximately 500 nm may bedrilled using a 775 nm femtosecond pulse laser. Geometrically speaking,it is possible to space these holes as close as 500 nm. When the holesare drilled one by one from one end to the other with the same laser,however, the previous method of laser machining a series of holes, thehole center-to-center spacing (pitch) cannot approach this limit.

The following example illustrates this problem. Assume that the firsthole is drilled with certain laser intensity and a certain number oflaser pulses. The laser intensity is chosen so that laser-inducedablation occurs only in the central portion of beam spot formed on thesurface, where the breakdown threshold is reached. This ablation leadsto hole drilling. Even though the surrounding area that is irradiateddoes not reach ablation threshold, however, it may undergo materialproperty changes that increase the ablation threshold for subsequentlaser irradiation. This phenomenon of laser irradiation-induced materialhardening, laser hardening hereinafter, means that using the same laserintensity and number of pulses on the hardened area, a new hole may notbe drilled in the laser hardened region. Therefore the hole-drillingreliability and reproducibility suffers. This issue is of particularimportance in a device, such as a photonic crystal, in which a largenumber of substantially identical holes placed with a precise sub-micronpitch are desired.

SUMMARY OF THE INVENTION

A solution to this problem is an exemplary laser machining process ofthe present invention, which allows closer placement of the holes toreach sub-wavelength center-to-center hole spacing (pitch).

The first step of this exemplary process is to separate the holepositions on the surface of the material sample into two groups selectedso that no two members of either group have a pitch less than the laserbeam spot size. Next the pulse energy of the laser beam is set to apredetermined level, selected to drill holes of the desired diameter inthe surface. Then the sample is positioned so as to focus the laser beamon the is surface at a hole position in the first group and a number oflaser pulses are applied to ablate the surface, thereby forming a holein the surface. The process is repeated for every hole position of thefirst group.

At this point the pulse energy of the laser beam is set to a secondpredetermined level, selected to drill holes of the desired diameter inthe surface once it has been laser hardened. Then the sample ispositioned so as to focus the laser beam on the surface at a holeposition in the second group and a number of pulses of the laser beamare applied to ablate the surface, thereby forming a hole in thesurface. Alternatively, the pulse energy of the beam may be maintainedat the same level and a greater number of pulses applied to the laserhardened surface. The process is repeated for every hole position of thesecond group.

Alternatively, the laser beam may be moved rather than the sample.

Another aspect of the present invention is an exemplary photonic crystalcomprising a plurality of holes formed in a material sample by themethod described above.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a top view drawing illustrating widely spaced holes and anassociated laser hardened region on a section of laser machined sample.

FIG. 1B is a top view drawing illustrating closely spaced holes and anassociated laser hardened region on a section of laser machined sample.

FIG. 2 is a block diagram illustrating an exemplary laser machiningapparatus of the present invention.

FIG. 3 is a top view drawing illustrating an exemplary photonic crystalformed by an exemplary laser machining method of the present invention.

FIG. 4 is a top view drawing illustrating a laser machined groove cut ina section of material using an exemplary laser machining method of thepresent invention.

FIG. 5 is a flowchart diagram showing an exemplary laser machiningmethod of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention describes a method used in laser material processingapplications for drilling small holes, having substantially uniform sizeand shape which holes have a diameter that is less than the wavelengthof the laser beam. Currently, many laser material processingapplications such as the machining of photonic band gap crystals usingfemtosecond laser pulses require sub-micron processing capability. FIG.1A shows a row of widely separated holes 102, which have been lasermachined into a material sample 100. These holes may be formed to have adiameter, d, less than the wavelength of the ultra-fast laser used todrill the holes. For example, if the laser used is a 775 nm femtosecondlaser, the holes in FIG. 1 may have a 500 nm diameter with a pitch, p,of 2 μm. Geometrically speaking, it is possible to space these holes asclose as 500 nm. However, when the holes are laser machined, the laserhardened regions 104 are also formed along the surface of the samplewith a diameter equal to the beam width, w. In the example, w is 1.5 μm.Because the material properties in the laser hardened regions may besignificantly different from those in the unhardened portions of thesurface, a hole drilled at least partially within the laser hardenedregions of the surface may have very different characteristics than onedrilled in an unhardened portion.

FIG. 1B illustrates an exemplary material sample in which laser machinedholes are closely spaced such that, if the holes are drilled one afteranother down the line, then each hole, after the first, has part of itsarea, near intersections 106, formed in a laser hardened region of thesurface. Using the same exemplary laser beam parameters as above withreference to FIG. 1A, the pitch, p, in FIG. 1B is 750 nm.

Holes formed in this manner may be incomplete or deformed, which isundesirable, particularly in applications, such as photonic crystals,where precise tolerances are desired. Because the minimum diameter ofthe laser hardened region is determined by the diffraction limited beamspot size of the laser, laser machining substantially identical holeswith a sub-wavelength pitch using a sequential drilling pattern may notbe possible in many materials.

FIG. 2 illustrates an exemplary embodiment of a laser machiningapparatus used in the present invention. The ultra-short pulse laser204, for example a 775 nm femtosecond laser, generates a laser beam 200,which desirably oscillates on a TEM_(0,0) mode. The laser beam may bepropagated between the components of the laser machining apparatusthrough the intervening air or along an optical fiber (not shown). Afterleaving the laser, the beam passes through a variable intensityattenuator and shutter assembly 206, which contains a shutter (notshown) and a variable intensity attenuator (not shown). The shuttercontrols the number of pulses and the variable intensity attenuatorcontrols the pulse energy of the beam. A half-wave plate and crossedpolarizers may form an exemplary variable intensity attenuator. Withthis combination, it is possible to select a desired laser intensity andnumber of pulses to be applied. An exemplary embodiment of the inventionuses an ultra-short pulse laser having a wavelength of 775 nm. With thevariable intensity attenuator, the pulse energy of the laser may be setbetween 1 nanojoule and 1 microjoule.

The laser beam is then focused onto the surface of the sample 100 to bemachined. An output coupler 202, such as a long-working-distance, highnumerical aperture microscope objective, may be employed to focus thebeam 200 onto the work piece 100. The sample 100 may be attached to aprecise XYZ-translation-stage 210 with nanometer resolution. By movingthe XYZ stage 210 relative to the optical coupler 202, one may preciselyfocus the laser beam to any spot on the sample, as illustrated by thefocused laser beam 208 in FIG. 2.

An exemplary method of the present invention, charted in FIG. 5, is aprocedure for drilling holes with sub-wavelength separations employing alaser machining apparatus of the sort illustrated in FIG. 2. This isaccomplished using a new drilling pattern.

The first step, 500, is to separate the positions on material sample 100where holes are to be drilled into two groups. These groups should bechosen so that no two holes in a given group are close enough to eachother that the laser hardened region formed during the machining of onehole will overlap part of the other hole. Mathematically this means thatthe separation between any two holes in the same group is at least equalto the width of a laser hardened region. For a line of holes the groupsmay usually be selected by numbering the holes sequentially with theeven numbered holes being one group and the odd-numbered hole being theother. It is contemplated that, if the holes of one group are still tooclose together after separating the holes into two groups, then threegroup may be created.

At the next step 502, the pulse energy of the laser beam is set to alevel that provides a laser beam intensity within the central area offocused spatial profile of the laser beam which is above the ablationthreshold of the unhardened material. The area of the laser beam profilein which the intensity exceeds the ablation threshold is desirably thesame as the area of one of the holes to be machined.

Next, in the exemplary embodiment shown in FIG. 5, the sample ispositioned such that the laser beam is focused on one of the first groupof hole positions on the surface of the sample, step 504. The sample isprecisely position in the X and Y directions to properly position thehole on the surface, and in the Z direction to focus the laser beam onthe surface. It is contemplated that the long working length microscopeobjective 202 may be manipulated to focus and/or position the laser beaminstead. It may be desirable for the laser beam to be propagated betweencomponents of the laser machining apparatus within an optical fiber inthe case when the microscope objective is moved to position the laserbeam.

A predetermined number of pulses are applied to ablate the surface 506.The number may be calculated to introduce a desired hole-size onnon-hardened area. Alternatively the hole drilling process may bemonitored in situ to ascertain when an appropriate number of pulses havepassed. In this way, very high accuracy in hole depth may be attained.

The decision at step 508 causes the previous two steps, 504 and 506, tobe repeated until all of the holes in the first group have been drilled.

Once all of the first positions on the surface have a hole, the processmoves to step 510. The pulse energy of the laser beam is set to a secondlevel that provides a laser beam intensity above the ablation thresholdof the laser hardened material. The area of the laser beam profile inwhich the intensity exceeds the ablation threshold is desirably the sameas the area of one of the holes to be machined.

Next, the sample is positioned such that the laser beam is focused onone of the second group of hole positions on the surface of the sample,step 512. The sample is positioned as in step 504.

The a number of pulses of laser beam are then applied to ablate thesurface 514. The number may be calculated to introduce a desiredhole-size on the laser hardened area or alternatively it may bemonitored as previously described with regard to step 506.

These last two steps, 512 and 514, may be repeated until all of theholes of the second group have been drilled. If there is a third groupof hole positions, the pulse energy may be increased again to accountfor the material being twice-exposed to the laser beam and the thirdgroup of holes drilled in a manner similar to the first two groups.

This method is applicable to any laser machining process where laserhardening of the surrounding areas may be present and may preventreliable drilling results when employing prior drilling sequences. Anexemplary implementation of the present invention, illustrated in FIG.3, involves drilling holes 102, with femtosecond pulses, in substrate300 to fabricate a photonic crystal. This substrate may be desirablyformed of a dielectric material or a multi-layer structure, such as asilicon-on-silicon dioxide structure. The exemplary photonic crystalillustrated in FIG. 3 has been formed as a two-dimensional photoniccrystal structure with the holes arranged in a square pattern. Thesquare pattern is only one possible pattern for a two-dimensionalphotonic crystal. Other patterns, such as a hexagonal pattern, may alsobe formed using the present laser machining method. It is contemplatedthat a one-dimensional photonic crystal structure may be formed usingthe laser machining process of the present invention, as well.

FIG. 4 illustrates an additional application of the exemplary lasermachining process described above with reference to FIG. 5. The materialsample 100 in FIG. 4 has had a groove 402 laser machined into itssurface. This groove may be cut according to the present invention byforming a series of sub-wavelength diameter holes having a pitch whichis a fraction of a hole diameter, so that the holes overlap forming asubstantially smooth groove. To form such a groove, it may be desirableto separate the hole positions into at least three groups.

While the invention has been described in terms of an exemplaryembodiment, it is contemplated that it may be practiced as describedabove within the scope of the appended claims. Also, it will beunderstood to one skilled in the art that a number of othermodifications exist which do not deviate from the scope of the presentinvention as defined by the appended claims.

What is claimed:
 1. A method to drill a plurality of holes in a materialusing an ultra-fast pulsed laser beam, comprising the steps of: a)setting a pulse energy of the laser beam to a first predetermined level,selected to provide an intensity greater than an ablation threshold ofthe material within a hole-drilling portion of the laser beam; b)positioning the material to focus the laser beam on one of a pluralityof first positions on a surface of the material; c) applying a number ofpulses of the laser beam to ablate the surface, thereby forming one ofthe plurality of holes in the surface; d) repeating steps b) and c)until all of the plurality of first positions on the surface have one ofthe plurality of holes; e) setting the pulse energy of the laser beam toa second predetermined level, selected to provide an intensity greaterthat a laser hardened ablation threshold of the material within thehole-drilling portion of the laser beam; f) positioning the material tofocus the laser beam on one of at least one second position on thesurface, wherein the one second position is between two adjacent ones ofthe plurality of first positions; and g) applying a number of pulses ofthe laser beam to ablate the surface, thereby forming one of theplurality of holes in the surface.
 2. A method according to claim 1wherein the at least one second position includes a plurality of secondpositions, each of the plurality of second positions being between twoadjacent ones of the plurality of first positions, the method furthercomprising the steps of: h) repeating steps f) and g) until all of theplurality of second position on the surface have one of the plurality ofholes; i) setting the pulse energy of the laser beam to a thirdpredetermined level, greater than the second predetermined level; j)positioning the material to focus the laser beam on one of at least onethird position on the surface, wherein the one third position is betweenone of the plurality of first positions and an adjacent one of theplurality of second positions; and k) applying a number of pulses of thelaser beam to ablate the surface, thereby forming one of the pluralityof holes in the surface.
 3. A method according to claim 1, furtherincluding the step of operating the laser such that the laser beamoscillates in the TEM_(0,0) mode.
 4. A method according to claim 1,wherein the laser beam has a wavelength and the step of setting thepulse energy of the laser beam to the first predetermined level includesthe step of setting the pulse energy of the laser beam to define thehole-drilling portion of the laser beam to have a diameter less than thewavelength of the laser beam.
 5. A method according to claim 4, whereinthe laser beam has a wavelength of 775 nanometers and the step ofsetting the pulse energy of the laser beam defines the hole-drillingportion of the beam to have a diameter of approximately 500 nm.
 6. Amethod according to claim 1, wherein the step of setting the pulseenergy of the laser beam to the second predetermined level includes thestep of setting the second predetermined level of pulse energy to alevel that is greater than the first predetermined level of the pulseenergy.
 7. A method according to claim 1, wherein the step of settingthe pulse energy of the laser beam to the first predetermined levelincludes the step of setting the pulse energy to be between 1 nanojouleand 1 microjoule.
 8. A method of drilling a plurality of holes in amaterial using a laser machining apparatus including an ultra-fastpulsed laser and an output coupler through which a laser beam passes,comprising the steps of: a. setting a pulse energy of the laser beam toa first predetermined level; b. positioning the output coupler to focusthe laser beam on one of a plurality of first positions on a surface ofthe material; c. applying a number of pulses of the laser beam to ablatethe surface, thereby forming one of the plurality of holes in thesurface; d. repeating steps b. and c. to position the output coupler atrespectively different ones of the first position until a portion of theplurality of holes have been formed at the plurality of first positions;e. setting the pulse energy of the laser beam to a second predeterminedlevel; f. positioning the output coupler to focus the laser beam on oneof at least one second position on the surface, wherein the one of theat least one second position is between two adjacent ones of theplurality of first positions; g. applying a number of pulses of thelaser beam to ablate the surface, thereby forming one of the pluralityof holes in the surface.
 9. A method to drill a plurality of holes in amaterial using an ultra-fast pulsed laser beam, comprising the steps of:a) setting a pulse energy of the laser beam to a predetermined level,selected to provide an intensity greater that an ablation threshold ofthe material within a hole-drilling portion of the laser beam; b)positioning the material to focus the laser beam on one of a pluralityof first positions on a surface of the material; c) applying a firstpredetermined number of pulses of the laser beam to ablate the surface,thereby forming one of the plurality of holes in the surface; d)repeating steps b) and c) until all of the plurality of first positionson the surface have one of the plurality of holes; e) positioning thematerial to focus the laser beam on one of at least one second positionon the surface, wherein the one second position is between two adjacentones of the plurality of first positions; and f) applying a secondpredetermined number of pulses, greater than the first predeterminednumber, to ablate the surface, thereby forming one of the plurality ofholes in the surface.
 10. A method according to claim 9 wherein the atleast one second position includes a plurality of second positions, eachof the plurality of second positions being between two adjacent ones ofthe plurality of first positions, the method further comprising thesteps of: g) repeating steps e) and f) until all of the plurality ofsecond position on the surface have one of the plurality of holes; h)positioning the material to focus the laser beam on one of at least onethird position on the surface, wherein the one third position is betweenone of the plurality of first positions and an adjacent one of theplurality of second positions; and i) applying a third predeterminednumber of pulses of the laser beam to ablate the surface, the thirdpredetermined number being greater than the second predetermined number,thereby forming one of the plurality of holes in the surface.
 11. Amethod according to claim 9, further including the step of operating thelaser such that the laser beam oscillates in the TEM_(0,0) mode.
 12. Amethod according to claim 9, wherein the laser beam has a wavelength andthe step of setting the pulse energy of the laser beam to thepredetermined level includes the step of setting the pulse energy of thelaser beam to define the hole-drilling portion of the laser beam to havea diameter less than the wavelength of the laser beam.
 13. A methodaccording to claim 12, wherein the laser beam has a wavelength of 775nanometers and the step of setting the pulse energy of the laser beamdefines the hole-drilling portion of the beam to have a diameter ofapproximately 500 nm.
 14. A method according to claim 9, wherein thestep of setting the pulse energy laser beam to the predetermined levelincludes the step of setting the pulse energy to be between 1 nanojouleand 1 microjoule.